1. Field of the Invention
The present invention relates to a thermal recording apparatus for information recording with uniform recording density on a thermal recording sheet by means of a thermal head.
2. Description of the Prior Art
In a conventional thermal recording apparatus, recording pulses of a same pulse width and a same voltage are supplied to the heating elements of the thermal head for dot recording. For this reason, in the recording of a belt-shaped image information which is a number of dots the same in number as the heating elements of the thermal head, the recording density tends to become lower at the start of recording and to fluctuate along said belt according to the instantaneous black level of image information.
Such fluctuation in the recording density along the belt-shaped image recording in the conventional apparatus results from a temperature change of the substrate of the thermal head corresponding to the black level of the image information, caused by the diffusion of heat generated in the heating elements at the image recording into said substrate. In the above-mentioned case, the temperature of the substrate shows an exponential change in the directiion of said belt.
Also, continued use of the thermal recording apparatus over a prolonged period often results in damage to the thermal head, due to overheating caused by the accumulated heat.
In order to prevent such destruction of the thermal head, a temperature sensor is mounted on or in the vicinity of the thermal head for gradually regulating the recording pulse width or the recording voltage which is supplied to the thermal head in the course of the image recording.
Such temperature control of the thermal head naturally leads to a fluctuation of the recording density, and a very costly detailed control has been indispensable in order to maintain such fluctuation of the recording density on the recorded image at a level which can not visually be discerned.